1. Field of the Invention
The present invention relates to ink jet printing, and, more particularly, to ink jet printing using color shingling to reduce visible printing defects.
2. Description of the Related Art
A typical ink jet printer forms an image on a print medium by ejecting ink from at least one ink jet printhead to form a pattern of ink dots on the print medium. Such an ink jet printer includes a reciprocating printhead carrier that transports one or more ink jet printheads across the print medium along a bi-directional scanning path defining a print zone of the printer. The bi-directional scanning path is oriented parallel to a main scan direction, also commonly referred to as the horizontal direction. The main scan direction is bi-directional. During each scan of the printhead carrier, the print medium is held stationary. An indexing mechanism is used to incrementally advance the print medium in a sheet feed direction, also commonly referred to as a sub-scan direction or vertical direction, through the print zone between scans in the main scan direction, or after all data intended to be printed with the print medium at a particular stationary position has been completed.
For a given stationary position of the print medium, printing may take place during one or more unidirectional scans of the printhead carrier. As used herein, the term xe2x80x9cunidirectionalxe2x80x9d will be used to refer to scanning in either, but only one, of the two bi-directional scanning directions. Thus, bi-directional scanning refers to two successive unidirectional scans in opposite directions. The term xe2x80x9cprinting swathxe2x80x9d will refer to the depositing of ink on the print medium during a particular unidirectional scan of the printhead carrier at which time individual printhead nozzles of the printhead are selectively actuated to expel ink. A printing swath is made of a plurality of printing lines traced along imaginary rasters, the imaginary rasters being spaced apart in the sheet feed direction.
Typically, each ink jet printhead will include a plurality of ink jet nozzles for expelling the ink. In ink jet printing, it is common to use the ink colors of cyan, magenta, yellow and black in generating color prints. Also, it is common in ink jet printing to have a single printhead having a dedicated nozzle array for each of cyan, magenta and yellow inks, respectively, wherein the three nozzle arrays are aligned vertically, that is, aligned in a direction parallel to the sub-scan direction.
In order to form the pattern of ink drops on the print medium, a rectilinear array, also known as rectilinear grid, of possible pixel locations is defined within the printable boundaries of the print medium. The center-to-center distance between pixels, sometimes referred to as dot pitch, is determined by the resolution of the printer. For example, in a printer capable of printing 600 dots per inch (dpi), the dot pitch of the array is one six-hundredth of an inch. The horizontal lines of the rectilinear array are the rasters, as introduced above.
The quality of printed images produced by an ink jet printer depends in part on the resolution of the printer. Typically, higher or finer resolutions, where the printed dots are more closely spaced, results in higher quality images. Increasing the resolution of an ink jet printer increases the number of dots to be printed in a unit area by the product of the increase factor in each dimension in the grid. For example, doubling the print resolution from 300 to 600 dpi in a rectilinear grid results in four times as many dots per unit area.
Printing quality using an ink jet printer of the type described above can be further improved by using a technique commonly referred to as shingling, or interlaced printing, wherein consecutive printing swaths are made to overlap. For example, in one known shingling mode using 50% shingling, approximately 50% of the dots for a particular color are placed on any given pass of the printhead, thereby requiring two passes of the printhead to completely print. The candidate dots of the first pass of the printhead are selected according to a checkerboard pattern. The remaining 50% of the dots are placed on a subsequent pass of the printhead.
When printing with an ink jet printer using a shingling method as described above, it is known to assign a particular interlace level to a tri-color printhead for use during printing on the print medium. For example, assuming that an image area corresponds to 16 rasters, it is known to assign a single interlace level for each of the cyan array of ink jetting orifices, magenta array of ink jetting orifices and yellow array of ink jetting orifices. It is also known to change the interlace level between portions of the image area which are spaced apart in the advance direction of the print medium. Finally, when using an ink jet printer having both a tri-color printhead and a black printhead, it is known to print using one interlace level for the tri-color printhead and a different or no interlace level for the black printhead. A selected one of a plurality of interlace levels may be used to effectively reduce a print artifact in a particular portion of an image area. For example, a 50% shingling technique (2 pass shingling) may be used to reduce a print artifact in one portion of the image area, while a 33% shingling technique (3 pass shingling) may be used to reduce a print artifact in a different part of the image area. Conventional methods of shingling would thus select the more restrictive 33% interlace level (3 pass shingling) for both portions of the image area so that all print artifacts are reduced. This may not be optimum from an efficiency standpoint in terms of throughput of the printer.
Traditional ink jet printers are designed to slightly underfeed the print medium in the sheet feed direction, essentially indexing a small amount short of the ideal index distance. This is done to hide indexing errors due to the mechanical system tolerances, as well as the algorithm used to control the indexing motion. The theory behind this approach is that a gap, observed as a white space on white paper, between consecutively printing swaths, due to a slight overfeed error, is more visible to the user than a slightly darker line due to an underfeed.
However, when shingling is employed, the errors due to purposeful underfeed can accumulate within a printed region, causing undesirable print defects. For example, a 0.15 percent underfeed using a printhead having 160 color nozzles having a nozzle pitch of one six-hundredth of an inch can yield a dot placement error just due to underfeed of around 10 micrometers. While this error is typically acceptable for single pass modes, where a 10 micrometers underfeed is desirable to mask other indexing errors, in shingled modes a print quality degradation is evident.
What is needed in the art is a printing method that reduces the visual print defects due to an underfeeding of the print medium.
The present invention reduces visual print defects, such as those occurring during ink jet printing due to an underfeeding of the print medium.
The invention, in one form thereof, relates to a method for reducing visual printing defects produced by an ink jet printer. The ink jet printer includes a reciprocating carrier that carries a first printhead nozzle array and a second printhead nozzle array along a bi-directional scanning path. The method includes the steps of providing the first printhead nozzle array spaced apart from the second printhead nozzle array in a main scan direction; defining a plurality of rasters for scanning during a first unidirectional scan of the first printhead nozzle array and the second printhead nozzle array; and printing on a print medium with both the first printhead nozzle array and the second printhead nozzle array to form printing lines traced along at least a portion of the plurality of rasters during the first unidirectional scan, wherein the first printhead nozzle array is controlled to print in a first shingling pattern during the first unidirectional scan and the second printhead nozzle array is controlled to print in a second shingling pattern during the first unidirectional scan.
In another form thereof, the present invention relates to an ink jet printer for printing on a print medium sheet. The ink jet printer has a printhead carriage unit including a reciprocating carrier that travels along a bi-directional scanning path in a main scan direction. A first printhead nozzle array is coupled to the carrier. A second printhead nozzle array is coupled to the carrier, the second printhead nozzle array being spaced apart from the first printhead nozzle array in the main scan direction. A controller is communicatively coupled to each of the printhead carriage unit, the first printhead nozzle array and the second printhead nozzle array. The controller executes instructions to effect printing on the print medium sheet during a first unidirectional scan of the carrier with both the first printhead nozzle array and the second printhead nozzle array to form printing lines traced along a plurality of rasters during the first unidirectional scan, wherein the first printhead nozzle array is controlled to print in a first shingling pattern during the first unidirectional scan and the second printhead nozzle array is controlled to print in a second shingling pattern during the first unidirectional scan.
In still another form thereof, the invention relates to a method of printing with an ink jet printer including a reciprocating carrier that carries a first printhead and a second printhead along a bi-directional scanning path. The method includes the steps of defining a raster corresponding to a print line to be printed on a print medium; scanning the first printhead and the second printhead across the print medium along the raster in a first unidirectional pass along the bi-directional scanning path; actuating the first printhead during the first unidirectional pass to selectively eject a first plurality of ink drops of a first color on the print medium along the raster to form a first plurality of ink dots of the print line; and actuating the second printhead during the first unidirectional pass to selectively eject a second plurality of ink drops of a second color, different from the first color, on the print medium along the raster to from a second plurality of ink dots of the print line, the first plurality of ink dots being interspersed with the second plurality of ink dots.